Volume 101, Issue 7, 01 April 2007
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
Effect of the properties of an intracavity heat spreader on second harmonic generation in vertical-external-cavity surface-emitting laser101(2007); http://dx.doi.org/10.1063/1.2714003View Description Hide Description
We investigate the effect of the properties of an intracavity diamond heat spreader on the lasing performance of vertical-external-cavity surface-emitting lasers. We compared two intracavity diamonds with different average birefringences. When a polarization selective element is inserted into the cavity, the infrared output powers are reduced by 13% and 7%, respectively, for the samples with higher and lower average birefringences. The pump-power limited maximum green output powers were changed from by rotating the samples with lower average birefringence. In order to explain the variation in green output power, the additional round-trip loss caused by diamondbirefringence was analyzed theoretically and experimentally. We found that the loss can be reduced when axes of the birefringence are aligned to the axes of the polarization selective element.
Forouhi-Bloomer and Tauc-Lorentz optical dispersions applied using spectroscopic ellipsometry to plasma-deposited fluorocarbon films101(2007); http://dx.doi.org/10.1063/1.2719271View Description Hide Description
Optical properties of the fluorocarbon (FC) films plasma deposited on Si substrates are evaluated in this work using multiple sample analysis (MSA)-based spectroscopic ellipsometry (SE) with representing the film optical constants by the Forouhi-Bloomer (FB) and Tauc-Lorentz (TL) optical dispersions. This SE analysis supported also with other film investigations results in a two-layer optical model consisting of an interface assimilated to FC species-permeated Si layer beneath a surface smooth, homogeneous, and isotropic FC bulk film. Both dispersions yield a low-dielectric constant quality visible range refractive index of 1.39 and almost identical model layer thicknesses. Deposition-dominated linear film growth is thus asserted. Specifically, the FB dispersion better describes the region near absorption cutoff with taking up a lower optical band gap (OBG) than that of the TL dispersion, indicating thereby that particular FC film absorptions included in the FB dispersion are excluded in the other. Also, the FB index spectrum tends to peak towards a broad maximum in the ultraviolet (UV) wavelength range, whereas this trend is absent in the TL index spectrum probably removed by the TL parameter cross correlations although reduced by MSA. Sample to sample analysis further shows that the TL parameter correlation is accentuated by the instrument-limited UV range. The FB parameters are much less affected by cross correlation such that even the sample to sample analysis may be readily used in conjuncture with the FB dispersion for thicker FC films above to monitor their index and OBG in tailoring desired film physical properties through deposition conditions.
101(2007); http://dx.doi.org/10.1063/1.2719009View Description Hide Description
Physical analysis of the long-wavelength room-temperature (RT) continuous-wave (cw) emission within the range from the GaAs-based GaInNAsSb/GaNAs quantum-well (QW) vertical-cavity surface-emitting diode lasers(VCSELs) is carried out. To this end, a comprehensive three-dimensional optical-electrical-thermal-gain self-consistent simulation of physical processes taking place inside a laser volume is applied. While the emission has been found to be reached practically without trouble, the emission needs additional optimizations. For the optimal QW structure, the emission in highly detuned VCSELs requires RT cw threshold currents more than 18 times higher than those of the one. Some threshold reduction may be achieved by adding the second quantum well. The cavity design has been proved to be optimized for the emission ensuring stable fundamental-mode operation even at elevated temperatures whereas the cavity is optimal for the emission for which threshold reduction is more important. For the emission, some structure optimizations have been proposed. The GaInNAsSb/GaNAs structure offers promising prospects to manufacture VCSELs emitting radiation, but some technology and construction improvements are still necessary.
101(2007); http://dx.doi.org/10.1063/1.2714683View Description Hide Description
From the symmetry analysis of frequency modulated (FM) stimulated Brillouinspectrum in glass, we demonstrate a separate detection of electrostrictive and absorptive FMBrillouinspectra as well as the detection of real and imaginary parts of Brillouinspectrum due to FM spectroscopy.
Optical characterization of liquid crystals by combined ellipsometry and half-leaky-guided-mode spectroscopy in the visible-near infrared range101(2007); http://dx.doi.org/10.1063/1.2714768View Description Hide Description
In this paper, we present our results for the anisotropic refractive index measurements of commonly used liquid crystal(LC)materials (E7 and 5CB) in the whole visible–near infrared range. In order to achieve a high accuracy in the obtained values, we have employed a combination of two techniques, namely, Mueller matrix spectroscopic ellipsometry in transmission and half-leaky-guided-mode (HLGM). Measurements with the HLGM technique are usually performed at a single wavelength. In order to obtain a spectroscopic measurement based on the HLGM technique in a wide wavelength range, we modified our commercial ellipsometer. In particular, we designed a sample holder by means of which measurements in guiding structures are also possible. Thus, we take advantage of the wide-spectrum light, emitted from a standard Xe lamp of the ellipsometer, also in the HLGM setup. The complementary and in-one-setup utilization of both techniques has allowed us to overcome most of the problems previously encountered in applying spectroscopic ellipsometry to the determination of the optical parameters of LCmaterials and eventually to achieve an accuracy in the obtained results of the order of . We discuss how individual elements of the Mueller matrix can be used to this aim. Finally, we compare the dispersion curves for the refractive indices and the birefringence of E7 and 5CB with the data already existing in the literature.
Derivation of a modified Fermi-Dirac distribution for quantum dot ensembles under nonthermal conditions101(2007); http://dx.doi.org/10.1063/1.2709614View Description Hide Description
Semiconductor quantum dot lasers offer significant advantages over traditional quantum well devices. However, the advantages due to the discrete density of states of a structure confined in all three spatial dimensions are usually not fully realized because of problems associated with the capture of carriers into the discrete states. In this paper we use a simple rate equation model to identify the processes that limit the performance of a quantum dot laser. This simplistic approach, while lacking the rigor of more complex models, allows us to develop a physical understanding of how the properties of the quantum dot electronic states effect the operation of a laser. The existence of a thermal, Fermi-Dirac distribution of carriers is shown to exist only when there are no recombination processes (either radiative or nonradiative). In a quantum well laser the rate of thermalization is much faster than the carrier loss processes and therefore the distribution appears to be close to Fermi-Dirac; however, in a quantum dot structure the slower capture/escape rates can cause nonthermal carrier distributions. The interplay of the radiative recombination and capture and escape rates in the dots is shown to define the mode of operation of the laser. An identity, derived simply in terms of the rates of carrier escape and spontaneous recombination and a confinement energy, predicts whether the carrier population is coupled across the dot ensemble. This will determine whether a semiconductor quantum dot laser exhibits single mode operation.
Mode identification of high-quality-factor single-defect nanocavities in quantum dot-embedded photonic crystals101(2007); http://dx.doi.org/10.1063/1.2714644View Description Hide Description
We investigate the quality factor and the mode dispersion of single-defect nanocavities based on a triangular-lattice GaAs photonic-crystal (PC) membrane, which contain InAs quantum dots(QDs) as a broadband emitter. To obtain a high factor for the dipole mode, we modulate the radii and positions of the air holes surrounding the nanocavity while keeping sixfold symmetry. A maximum of 17 000 is experimentally demonstrated with a mode volume of . We obtain a of , one of the highest values ever reported with QD-embedded PC nanocavities. We also observe ten cavity modes within the first photonic band gap for the modulated structure. Their dispersion and polarization properties agree well with the numerical results.
101(2007); http://dx.doi.org/10.1063/1.2715839View Description Hide Description
An approach fully based on the rf magnetron sputtering of a pure silica target has been developed to deposit a siliconnanocrystal based optical planar microcavity. Tunable narrow emission bands, directionality of the radiation pattern along the normal of the cavity surface, and resonant enhancement of the luminescence intensity have been demonstrated. Time resolved photoluminescence measurements also confirmed an increase of the spontaneous emission rate of siliconnanocrystals when they are embedded inside the optical resonator. These results are attributed to the anisotropic redistribution of the optical density of states induced by the resonator.
101(2007); http://dx.doi.org/10.1063/1.2715752View Description Hide Description
Studies of electron emission by photoelectric process from pure Zn cathodes by UV laser beams were performed. As UV sources, two different wavelength excimer lasers were utilized in order to investigate the photoemission as pure photoelectric process and multiphoton process. The Schottkyeffect was also considered. The utilized laser sources were a KrF operating at wavelength , full width at half maximum (FWHM), and a XeCl operating at wavelength , FWHM. The cathodework function was , a value lower and higher than the photon energies of KrF and XeCl lasers, respectively. The photocathodes were tested in a vacuum photodiode cell at . The cathodes were irradiated at normal incidence and the anode-cathode distance was set to . Due to the electrical breakdown into the photodiode gap, the maximum applied accelerating voltage was . Under the above experimental conditions a maximum of electric field resulted. Under the space charge effect, the electron emission was higher than the one expected by the Child-Langmuir law. In saturation regime the electron emission increased with the accelerating voltage owing to the Schottkyeffect and plasma formation. The highest output current was achieved with the KrF laser at laser energy. Its value was about , corresponding to a global quantum efficiency of approximately , while the temporal quantum efficiency presented a maximum value of located at the tail of the laser pulse. The estimated efficiencies with the XeCl laser were lower than the KrF ones as well as the output current and the plasma formation, although higher energy values than the KrF ones were utilized. By the results obtained, we conclude that the plasma generation is strongly due to the extracted current but weakly to the used laser intensity.
101(2007); http://dx.doi.org/10.1063/1.2359117View Description Hide Description
We report on the synthesis, characterization, and physical properties of modified polymeric matrices incorporating silicon atoms in their structure and doped with laser dyes. When the silicon-modified organic matrices incorporated pyrromethene 567 (PM567) and pyrromethene 597 (PM597) dyes as actual solid solutions, highly photostable laser operation with reasonable, nonoptimized efficiencies was obtained under transversal pumping at . At a pump repetition rate of , the intensity of the laser emission remained at the level or above the initial lasing intensity after 100 000 pump pulses in the same position of the sample, corresponding to an estimated accumulated pump energy absorbed by the system of 518 and for PM567 and PM597, respectively. When the pump repetition rate was increased to , the laser emission of dye PM567 decreased steadily and the output energy fell to one-half its initial value after an accumulated pump energy of 989 GJ/mol. Dye PM597 demonstrated a remarkable photostability, and under pumping the laser emission from some samples remained stable after 700 000 pump pulses in the same position of the sample, corresponding to an accumulated pump energy of . Narrow linewidth operation with tuning ranges of up to was obtained with both pyrromethene dyes when some of the samples were incorporated into a grazing-incidence grating oscillator.
101(2007); http://dx.doi.org/10.1063/1.2711396View Description Hide Description
Unlike at any single TEM (, ) mode laser, there is a subluminous phase velocity region located along the central region of a TEM (0,1) mode laser. In conjunction with the high longitudinal electric field in this region, it forms another acceleration channel, which also locates inside the transverse ponderomotive potential trap. Through simulation, it is found that relativistic electrons injected into this acceleration channel can stand at the acceleration phase for a long time and be synchronously accelerated to high energies. Also, the accelerated electrons can be well confined inside the trap avoiding the transverse scattering problem.
- PLASMAS AND ELECTRICAL DISCHARGES
Increasing lifetime of the plasma channel formed in air using picosecond and nanosecond laser pulses101(2007); http://dx.doi.org/10.1063/1.2713997View Description Hide Description
We report experiments on a pump-probe configuration to elucidate the formation of a plasma channel by the hydrodynamic evolution of air breakdown in laser focus. A stable air breakdown was produced by focusing a picosecond laser pulse to create a shock driven plasma channel in the laser focus for propagating a nanosecond pulse. A four fold increase in the lifetime of the channel estimated by monitoring the temporal evolution of the fluorescence of a spectral line at of transition is reported. Assuming plasma in local thermal equilibrium plasma temperature of and an electron density of were determined using a Stark broadening of line of transition in the channel. An enhancement in the electron density of the plasma channel was observed at the delay of the nanosecond laser pulse relative to the picosecond laser pulse.
101(2007); http://dx.doi.org/10.1063/1.2714677View Description Hide Description
Tomography of a plasma enables the distribution of electron density to be visualized. We report on the design of two tomographic interferometer systems used to measureplasma electron density distributions in a high-current pulsed cathodic vacuum arc. The method is shown to be capable of microsecond time resolution. The spatial resolution of the quasioptical interferometer operating at 2 mm wavelength is 20 mm and the spatial resolution of the waveguide-based interferometer operating at 8 mm wavelength is 50 mm. In both cases the resolution achieved depends on the launching and receiving geometries. We developed criteria for assessing the tomogram for artifacts arising from limited sampling. First results of the spatial and temporal history of plasma in a high-current vacuum arc guided by a curved magnetic filter are presented and indicate poloidal field fluctuations reminiscent of magnetohydrodynamic instabilities in pinches. The applicability of the tomographic interferometry method to optimize plasma transport through the filter is also demonstrated.
An alternative method for gas temperature determination in nitrogen plasmas: Fits of the bands of the first positive system101(2007); http://dx.doi.org/10.1063/1.2537448View Description Hide Description
A method of gas temperature determination in nitrogen or nitrogen doped discharges is presented. The method employs fits of numerically generated spectra of the 0-0, 1-0, and 2-0 bands of the first positive system of nitrogen to experimental measurements. Excellent agreement between gas temperature values inferred by using this method and by using the 3-0 band peak ratio method [M. Simek and S. De Benedictis, Plasma Chem. Plasma Proc.15, 451 (1995)] is demonstrated for a helicon plasma. The spectral model is available for use by the plasmaspectroscopy community. The model, along with user instructions, can be downloaded from Electronic Physics Auxiliary Publication Service of American Institute of Physics. The model includes the line positions, Hönl-London factors, and provides rapid determination of gas temperature if one or more of the aforementioned emission rovibrational band spectra are available.
Photonic bands in two-dimensional microplasma arrays. I. Theoretical derivation of band structures of electromagnetic waves101(2007); http://dx.doi.org/10.1063/1.2713939View Description Hide Description
Two theoretical approaches appropriate for two-dimensional plasma photonic crystals reveal dispersions of propagating waves including photonic (electromagnetic) band gaps and multiflatbands. A modified plane-wave expansion method yields dispersions of collisional periodical plasmas, and the complex-value solution of a wave equation by a finite difference method enables us to obtain dispersions with structure effects in an individual microplasma. Periodical plasma arrays form band gaps as well as normal photonic crystals, and multiflatbands are present below the electron plasma frequency in the transverse electric field mode. Electron elastic collisions lower the top frequency of the multiflatbands but have little effect on band gap properties. The spatial gradient of the local dielectric constant resulting from an electron density profile widens the frequency region of the multiflatbands, as demonstrated by the change of surface wave distributions. Propagation properties described in dispersions including band gaps and flatbands agree with experimental observations of microplasma arrays.
Photonic bands in two-dimensional microplasma arrays. II. Band gaps observed in millimeter and subterahertz ranges101(2007); http://dx.doi.org/10.1063/1.2713940View Description Hide Description
Plasma photonic band gaps have been observed in a two-dimensional microplasma array, and we have characterized their properties by both experimental and theoretical results. Microplasma columns ignited in helium near atmospheric pressure formed crystal-like structures in a square lattice with a lattice constant from . Microwaves in the millimeter range transmitting through the array region attenuated at frequencies of photonic band gap in the direction, as predicted by the modified plane-wave expansion method. Frequency dependence around the band gap was clarified in the numerical analysis of electromagnetic wave propagation and agreed with experimental results. Electron density in microplasmas was estimated to be from the attenuation rate at the band gap in the direction. Variation of the lattice constant induced frequency shift of the band gap in the millimeter and subterahertz regions, and so plasma photonic crystal can perform as a dynamically controllable band-stop filter.
Nonthermal atmospheric rf plasma in one-dimensional spherical coordinates: Asymmetric sheath structure and the discharge mechanism101(2007); http://dx.doi.org/10.1063/1.2715745View Description Hide Description
We present one-dimensional simulations of atmospheric pressure rf-excited plasma with two concentric spherical electrodes and the inner electrode powered. The gas used is helium with 0.1% nitrogen addition. The gap distance between the inner and outer electrodes is . The coupled continuity equations and electron energy equation are solved with Poisson’s equation using the finite element method. A mode transition is observed in the discharge power-voltage curve between 1 and . In the low power mode, ionization rate peaks only near the inner electrode. The electron-impact excitation and ionization rates peak in the local cathodic phase. In the high power mode, the rate of ionization peaks near the outer electrode as well as the inner electrode. The inner sheath significantly shrinks and the direct electron-impact ionization is the primary ionizationreaction near the inner electrode. The ionization rate near the outer electrode is due to Ohmic sheath oscillation heating of electrons, resulting in a peak in metastable helium creation. Penning ionization is the major ionizationreaction near the outer electrode. Thus, two different ionization mechanisms coexist near the inner and outer electrodes. Electron heating near the outer electrode may have implications for surface processing in atmospheric pressure microdischarges. The local field approximation (LFA) in high power mode fails to predict the ionization rate peak near the outer electrode due to its inability to properly account for electron diffusion in the presence of both a strong electric field and electron density gradient. However, use of the LFA is adequate to model the low power mode and it correctly predicts the existence of the mode transition.
101(2007); http://dx.doi.org/10.1063/1.2720093View Description Hide Description
The cylindrical Hall thruster (CHT) is an attractive approach to achieve a long lifetime thruster operation especially in low power space applications. Because of the larger volume-to-surface ratio than conventional coaxial Hall thrusters, the cylindrical Hall thrusters are characterized by a reduced heating of the thruster parts and potential lower erosion. Existing CHTs can feature a short coaxial channel in order to sustain a high ionization in the thruster discharge. A 5.6 cm diameter cylindrical Hall thruster was developed and operated with and without a short coaxial region of the thruster channel, in the power range of 70–300 W. It is shown that the CHT without coaxial region can operate stable and achieve higher thrust efficiency, 22%–32% more than that with a coaxial region. Plasma probe measurements inside the thruster channel and ion energy measurements in the plasma plume suggest that the ionization/acceleration region in the CHT is located near the anode region where a radial magnetic field is stronger.
Probe diagnostics of argon-oxygen-tetramethyltin capacitively coupled plasmas for the deposition of tin oxide thin films101(2007); http://dx.doi.org/10.1063/1.2561749View Description Hide Description
Langmuir probemeasurements in nondepositing and depositing rf capacitively coupled (CCP) plasmas are briefly reviewed and compared to the results obtained in our rf system used for the deposition of tin oxide thin films from argon-oxygen-tetamethyltin plasmas. Typically in our experimental conditions for tin oxide deposition, values of and were measured. These values are consistent with those generally reported in other depositing discharges. The shape of the electron energy probability function (EEPF), obtained from the Druyvesteyn procedure, was discussed too. As a consequence of the two electron heating mechanisms in capacitively coupled discharges, that is, ohmic and stochastic heating, the electrons have a bi-Maxwellian EEPF at low pressure (in the range of ). Moreover, a deep “hole” appears in the EEPF at the energy which could correspond to the resonant peak of the vibrational excitation cross section of some molecules which can be present in the discharge, such as , , or CO.
101(2007); http://dx.doi.org/10.1063/1.2715845View Description Hide Description
Optical emission spectroscopy as a noninvasive plasma diagnostic was employed to study mode transitions and hysteresis in an inductively coupled plasma in Ar and mixtures. Using selected Ar lines, basic plasma parameters, relevant to the analysis of the mode transitions, were evaluated. Small changes of the electron energy distribution function in the vicinity of the mode transition were detected. The role of metastable Ar atoms in mode transitions and in a hysteresis was clarified. Enhanced production of metastables in the hysteresis region as well as faster transitions in plasmas with higher influence of metastables were observed.